Refined Sea Salt Markers for Coastal Cities Facilitating Quantification of Aerosol Aging and PM_2.5 Apportionment

Abstract

Sea salt (ss) aerosols in PM 2.5 are often quantified through source apportionment by applying sodium (Na + ) and chloride (Cl − ) as the markers, but both markers can be substantially emitted from anthropogenic sources. In this study, we differentiate ss from nonss (nss) portions of Na + and Cl − to better apportion PM 2.5 in a coastal tropical urban environment. Size-resolved ionic profiles accounting for Cl − depletion of aged ss were applied to 162-day measurements during 2012 and 2018− 2019. Results show that the nss (likely anthropogenic) portions, on average, account for 50−80% of total Na + and Cl − in submicron aerosols (PM 1 ). This corresponds to up to 2.5 μg/m 3 of ss in submicron aerosols that can be ∼10 times overestimated if one attributes all Na + and Cl − in PM 1 to ss. Employing the newly speciated ss-and nss-portions of Na + and Cl − to source apportionment of urban PM 2.5 via positive matrix factorization uncovers a new source of transported anthropogenic emissions during the southwest monsoon, contributing to 12−15% of PM 2.5 . This increases anthropogenic PM 2.5 by ≥19% and reduces ss-related PM 2.5 by >30%. In addition to demonstrating Cl − depletion (aging) in submicron aerosols and quantifying ssNa + , nssNa + , ssCl − , as well as nssCl − therein, the refined PM 2.5 apportionment resolves new insights on PM 2.5 of anthropogenic origins in urban environments, useful to facilitate policy making.